Use of a novel relationship between bond length and bond order to calculate accurate bond orders for carbon–carbon bonds

An exact relationship between bond length and bond order has been derived for the first time based on the concept of electron density. This relationship allows the calculation of sufficiently accurate bond orders and also determines the number of bond-forming electrons. According to this novel relationship between bond order and bond length, the bond order of the carbon–carbon bond in ethylene is 1.75, whereas it is 2.50 in acetylene. These bond orders are readily interpreted by the fragmentation of π-bonds and a consequent decrease in bond order, which is further supported by the chemical properties of these molecules. Assuming structure-specific fragmentation of π-bonds (i.e. one structural motif always adheres to one or two types of bond fragmentation scheme), the bond orders can be predicted for molecules containing multiple carbon–carbon bonds in excellent agreement with the experimental findings.